1. Field of the Invention
The present invention relates to a drive circuit for a vibration wave motor.
2. Related Background Art
In the past, in order to control a speed of a vibration wave motor, a drive frequency is changed or a drive voltage is changed.
In the former method, in order to control the speed over a wide range, the drive frequency must be changed from a high frequency far from a resonance frequency of the motor. On the other hand, the drive efficiency of a vibration wave motor is high for a predetermined range of frequencies near the resonance frequency, but when the motor is driven at a frequency far from the near-resonance frequency, the drive efficiency decreases. Thus, it is difficult to drive the motor at a high efficiency over the entire speed range.
In the latter method, a power supply which varies over a wide range is required and a power supply circuit is complex and of large size.
Thus, it may be proposed to change a combination of drive frequency and voltage to control the speed over a wide range in a frequency band which permits a high drive efficiency. However, it is not possible to continuously change the motor speed by simple combination of the frequency and the voltage.
For example, the frequency is changed from a first frequency to second frequency at a first voltage to set the motor speed to a first speed at the first frequency and to a second speed at the second frequency so that the motor speed is controlled between the first and second speeds by the combination of the first voltage and the first to second frequencies, and when the voltage is changed to a second voltage level, the motor speed is changed from the second speed to a third speed by the above frequency change at the second voltage. In this case, the following problem arises at the voltage switching point.
If the frequency is changed to the second frequency under the first voltage level to change the motor speed to the second speed and then the voltage is changed to the second voltage level to shift the motor speed toward the third speed, the voltage is set at the second level and the frequency is set at the second frequency. As a result, the motor speed is switched from the second speed to the third speed by the switching of the voltage level and a continuous change from the second speed to the third speed is not attained.